I dug out these front brake backing plates for a user on the Opel Manta Owners Club Forum, but his interest appears to have waned. They were saved from an Opel Manta B series (i200 model) that was split for spares many years ago and they have lived in my loft ever since.

Manta brake disc back plates - maximum diameter is 280mm

I ran over them with a fibre brush on my drill to see if there were part numbers visible under the dirt. As the photos will show, there was some surface rusting at the centre, but nothing that appears to have penetrated the metal.

Manta brake disc back plates -  mounting holes are asymmetrical

Their maximum diameter is 280mm, the “bell” section is 75mm across and the hole in the centre is 48mm. Note the 3 mounting holes are in an asymmetrical arrangement.
If any one else should be interested in these brake backing plates, you can have them for £25 posted to the UK.

Manta brake disc back plates - the “bell” section is 75mm across

Manta brake disc back plates - the centre is 48mm

Manta brake disc back plates - some rusting at the centre

UPDATE:

14/06/2014 - Now Scrapped